1. Field of the Invention
The present invention relates to a frequency or speed display device, and more particularly to a digital frequency meter, speedometer or tachometer wherein the display is updated according to the magnitude of a sensed frequency or speed change.
2. Detailed Description of the Prior Art
Recently, a digital display has come to be used as a speedometer or tachometer in automotive vehicles instead of a conventional analog pointer type meter. In a digital display type meter, a binary coded decimal (BCD) signal representing the vehicle or engine speed is latched for each of the pulse durations of a clock signal and sent through a decoder for conversion to a seven segment signal. This signal is then sent to a digital display such as a fluorescent display tube using seven segment digit displays, thereby indicating the vehicle or engine speed.
If the latch pulse duration is too short, the display tends to flicker because of slight speed fluctuations and power supply voltage changes even while the vehicle is running at substantially constant speed. In order to avoid this problem, prior art devices have adopted a relatively long pulse duration, for example of about two seconds, for updating the speed display, by using a clock signal of relatively low frequency.
However, with such a display method, the display of the vehicle speed is updated relatively infrequently even when the vehicle or engine speed changes rapidly, as during acceleration or braking. Therefore, a lag occurs between the actual vehicle speed and the displayed speed so that the driver does not get accurate and up to date information.